In precision instrumentation, and particularly in electro-optical precision instrumentation, and especially in spectrophotometers which may be used for purposes such as optical chemical analysis, it is extremely important to avoid or eliminate noise interference signals from electrical measurement signals. Some of the most persistent and serious sources of such noise and interference are related to the alternating current power supply. For instance, ambient room illumination from gaseous discharge lamps such as fluorescent lamps varies substantially in response to the alternations of the power frequency. Similarly, the fluctuations in the voltage of the power supply to the spectrophotometer cannot be completely filtered out, and may appear in the output signal. Also, ambient acoustic signals which are power frequency related can arise from electrical apparatus which is in operation near the spectrophotometer, and which can create minute mechanical movements of optical components which give rise to further power frequency-related noise.
Accordingly, it is an important object of the present invention to provide an analog-to-digital converter system which eliminates power line frequency related noise and interference signals from an analog voltage signal as converted to a digital signal.
It is known that power line frequency noise signals can be eliminated by selecting a conversion interval for an analog-to-digital converter which corresponds to the cycle interval of the power line or to exact multiples of that interval. Accordingly, the selection of conversion intervals corresponding to the power line cycle interval is advocated. However, different power line frequencies are employed such as 50 cycles as well as 60 cycles. Furthermore, the frequency of the power line may be intentionally or unintentionally varied from a strictly regulated 60 cycle or 50 cycle frequency. For instance, in some communities in the world it is common to increase the power line frequency after a power shutdown in order to readjust the customer's clocks. In other systems, such as small independent power systems, the frequency of the power system is not carefully regulated. Accordingly, it is not possible to achieve complete and reliable power line frequency related noise elimination from the signal being processed by simply determining the conversion interval as a fixed interval corresponding to the nominal power line cycle interval.
Accordingly, it is another important object of the present invention to provide an analog-to-digital converter system which will automatically eliminate power line related noise from the signal being converted no matter what the power line frequency may be, and no matter how the power line frequency may vary.
Further objects and advantages of the invention will be apparent from the following description and the accompanying drawings.